Poly(acrylonitrile) (PAN) is a synthetic, semi crystalline organic polymer that typically has a linear structure of general formula (C3H3N)n. Commercially, PAN is often produced in the form of a co-polymer with one or more other ethylenically unsaturated monomers.
Although PAN-based polymers are generally thermoplastic, they may not go through a molten transition under normal conditions, but rather degrade prior to melting.
PAN-based polymers are particularly versatile and are used to manufacture numerous products including filtration membranes, and fibres having a diverse range of applications.
PAN-based fibres have been found to be particularly suited for use in the manufacture of carbon fibre. This typically involves first thermally oxidising PAN-based fibre in air to form oxidised PAN fibre which is then carbonised at high temperature in an inert atmosphere to make the carbon fibre.
The properties of carbon fibre, such as high stiffness, high tensile strength, low weight, high chemical resistance, high temperature tolerance and low thermal expansion, make it particularly suitable for use in aerospace, civil engineering, military, automotive and sporting applications.
In use, carbon fibres are typically combined with a polymer resin to form a composite structure. The resulting composite structures are renowned for having a very high strength-to-weight ratio.
Because of its unique properties, PAN-based polymer is particularly well suited for use in the manufacture of carbon fibre. Despite being used for many years as a precursor material in the manufacture of carbon fibre, those skilled in the art will be aware that acrylonitrile presents numerous challenges in the manufacture of PAN-based polymers. In particular, due to the high reactivity of acrylonitrile and the poor solubility of PAN-based polymers, the controlled polymerisation of acrylonitrile has presented a significant challenge to polymer scientists.
PAN-based polymers have traditionally been produced by conventional free radical polymerisation, the process of which offers limited control over the molecular weight and dispersity of the resulting polymer.
Increasing the molecular weight while maintaining a low dispersity of PAN-based polymers is believed to play an important role in enhancing certain properties of products, such as carbon fibre, derived from the polymer.
Accordingly, considerable research effort has to date been directed toward improved methodology for producing PAN-based polymer.
Anionic polymerisation techniques have been applied with some success to produce relatively well-defined PAN-based polymer. However, the techniques employed require relatively harsh polymerisation conditions that would present as major limitations to adopting the technology commercially. Furthermore, the technique has only provided for a relatively modest gain in molecular weight over other known techniques.
In more recent times, considerable attention has focussed on using so called living or controlled radical polymerisation techniques to prepare PAN-based polymers. The use of such techniques has resulted in an ability to produce PAN-based polymers with an increase molecular weight and a relatively low dispersity. For example atom transfer radical polymerisation (ATRP) has been used to prepare PAN with a molecular weight (Mn) of about 120,000 g/mol and a dispersity (Mw/Mn) of about 2 (Journal of Polymer Science Part A: Polymer Chemistry Volume 51, Issue 2, pages 340-346, 2013).
Despite offering improvements in the preparation of PAN-based polymers, most of the ATRP techniques developed to date inherently introduce transition metal residues into the resulting polymer. The presence of such transition metal residues can be detrimental in certain applications for the polymer, for example in the manufacture of carbon fibre.
Other living/controlled radical polymerisation techniques have also been applied with some success. For example, Reversible Addition-Fragmentation chain Transfer (RAFT) polymerisation has been employed in the manufacture of the PAN-based polymers. For example, producing PAN with a Mn of about 33,000 g/mol and a dispersity of 1.29 by RAFT polymerisation was considered to be a significant achievement (Journal of Polymer Science Part A: Polymer Chemistry Volume 45, Issue 7, pages 1272-1281, 2007). In another example, producing PAN with a Mn of 200,000 g/mol and a dispersity of 1.7-2.0 by RAFT polymerisation was also claimed as a significant progress (European Polymer Journal, Volume 44, Pages 1200-1208, 2008).
Those skilled in the art will appreciate that as the Mn of a given polymer increases it becomes increasingly difficult to maintain a low dispersity. In the manufacture of PAN-based polymers it has proven difficult to not only produce polymers having a Mn of greater than 100,000 g/mol but also to maintain the dispersity of the polymer below about 1.35. In such an environment an ability to produce PAN-based polymers with only a modest increase in Mn while maintaining a low dispersity is considered in the art to be a significant achievement.
Accordingly, there remains an opportunity to develop PAN-based polymers that exhibit improved properties such as increased molecular weight with low dispersity.